Many plantation softwood species, generally members of the Pinaceae family, and certain hardwoods lack natural durability and are subject to rapid degradation by a host of insects and microorganisms that play a vital role in lignocellulosic mineralization and nutrient recycling processes in nature. Thus it is common practice to protect timber and other wood products using insecticides and fungicides.
The wood degrading microflora include bacteria and a large number of fungi of which the latter are the main cause of sapstain, mould and decay. Sapstain fungi and moulds produce unsightly superficial effects whereas decay fungi (also called rot fungi) severely weaken or completely break down wood.
White rot fungi are so called because they efficiently degrade all three principal wood cell wall biopolymers cellulose, hemicellulose and lignin, eventually producing a bleaching effect. Brown rot fungi are capable of utilising the cellulose and hemicellulose components, but only modifying the lignin component slightly, resulting in a darkening effect. Dry rot fungi are essentially brown rot fungi that can transport water into wood and decompose it slowly. A further category, the soft rot fungi, utilise cellulose and hemicellulose like brown rot fungi but their growth is restricted to inside the woody cell wall and the resulting damage becomes evident more slowly than with other decay fungi. Further information on wood decay fungi and their modes of action can be found in F. W. M. R. Schwarze “Wood decay under the microscope”, 2007, Fungal Biology Reviews doi:10.1016/j.fbr.2007.09.001.
Traditional preservation systems include inorganic preservatives such as copper chrome arsenic, sodium octaborate and alkaline copper quaternary (“ACQ”) that are introduced into timber in an aqueous medium, or carbon-based preservatives delivered in a non-aqueous medium termed a light organic solvent preservative (“LOSP”). These approaches achieve moderate levels of preservative penetration and result in generally effective protection but are expensive because they use vacuum and pressure to a greater or lesser extent, and must be performed in a separate step to other wood processing operations, thus incurring additional cost. Inorganic preservatives are loosing favour because of toxicity issues and the accompanying preservation methods require special care to avoid problems with dimensional stability. Persistent solvent residue problems characterise LOSP systems.
In recent years carbon-based preservatives and boron compounds have been incorporated into the glues and resins used to make engineered wood products, such as plywood and laminated veneer lumber, and reconstituted wood products such as particle board, oriented strand board and the like. While this system may be integrated easily into veneer and fibre lay up operations, it does require the preservative to be stable under extreme conditions such as heat (up to 250° C.), pressure and/or high pH (9-13) commonly encountered during layup and hot pressing of the wood product.
Glueline addition of biocides presents other challenges. The addition must not significantly change the working properties of the resin (viscosity, dynamic surface tension, etc) that affect pumping, curtain coating onto veneers or mixing with fibres, as well as other properties including wet bond strength (tack), curing rate and the like. Most importantly, the addition must not reduce the bond strength of the final product. It is therefore imperative to add the least possible amount of biocide to the adhesive that will prevent decay during the service life of the glued wood product.
Commercially significant decay fungi are dominated by the brown and white rots, some of which are susceptible to triazole fungicides such as tebuconazole and propiconazole. Tebuconazole and propiconazole are widely used in LOSP processes. However, unworkably high addition rates are required to achieve efficacy when tebuconazole and/or propiconazole are applied to the glues and resins used to make engineered and reconstituted wood products.
The effectiveness of a wood preservative in the field is limited by any point of weakness against individual fungal species. Accordingly, there is a need with respect to glueline application to increase triazole addition rates by some means, or in some other way overcome the weaknesses of triazole fungicides against certain fungi that can be problematic with glued wood containing products. It is also desirable to increase the efficacy against dry rots and soft rots. At the same time, health, safety and environment considerations are among factors driving a reduction in biocide concentrations in preservative-treated wood products.
The triazoles under consideration in this disclosure are summarised in Table 1. Triadimenol is the carbonyl reduced form of triadimefon. Triadimefon and triadimenol have similar physical properties apart from melting point and vapour pressure, and contain one and two chiral carbons, respectively. Cyproconazole has similar physical properties to tebuconazole and propiconazole but importantly differs in the octanol-water partition coefficient (Kow) and air-water distribution coefficient (Henry's law constant). These azoles contain two, one and two chiral carbons, respectively (Table 1). All of the listed triazoles are therefore present as various stereoisomeric mixtures.
The formulation types available for agricultural use (Pesticide Manual, 15th Edition, 2009) reflect in large part the ease of manufacture which is based on the different physical properties of the triazoles, their stability in different formulation types, and the crops and disease control profiles of these active ingredients. Accordingly triadimefon is formulated as powders, granule, paste or emulsifiable concentrate; triadimenol is formulated in a wide range including powders, granule, emulsions, suspensions, and concentrates. Cyproconazole is formulated as a concentrate, solution or granule; tebuconazole as a very wide range including powders, granule, emulsions, gel, suspensions, and concentrates. Propiconazole, which as a technical is a liquid at ambient temperature, is formulated as an emulsion or gel.
TABLE 1Triazole structures and selected properties (The Pesticide Manual, 15th Edition, 2009).CompoundTriadimefonTriadimenolCyproconazoleTebuconazolePropiconazoleStructure StereochemistryRacemate70% A, 30% BRacemateRacemate60% cis, 40% transMolecular weight293.8295.8291.8307.8342.2FormColourless crystalsColourless odourlessColourless solidColourless crystalsYellow odourlessweak char. odourcrystalsviscous liquidMelting point82.3° C.A 138.2° C., B 133.5° C.106.2-106.9° C.105° C.−23° C. (glasstransition temp)Vapour pressure0.06 mPa0.0006 mPa0.026 mPa0.0017 mPa0.027 mPaKow (log P)3.11A 3.08, B 3.283.13.73.72Henry (P m3 mo1−1)9 × 10−5A 3 × 10−6, B 4 × 10−62.6 × 10−41 × 10−59.2 × 10−5Sol. Water64 mg/L (20° C.)A 62 mg/L (20° C.)B93 mg/L (22° C.)36 mg/L (20° C.)100 mg/L (20°)33 mg/L (20° C.)Sol. DCM>200 g/L (20° C.)>250 g/L (20° C.)430 g/L (25° C.)>200 g/L (20° C.)Completely miscSol. toluene>200 g/L (20° C.)20-50 g/L (20° C.)100 g/L (25° C.)50-100 g/L (20° C.)Completely miscSol. hexane6.3 g/L (20° C.)0.1-1.0 g/L (20° C.)1.3 g/L (25° C.)<0.1 g/L (20° C.)47 g/L (25° C.)StabilityStable to hydrolysis,Stable to hydrolysis,Stable in water 35 dStable to hydrolysis,Stable to 320° C., noDT50 >1 y pH 4,7,9DT50 >1 y pH 4,7,9at 50° C. pH 1-9DT50 >1 y pH 4-9sign. hydrolysis
The principal mode of action of triazole fungicides is inhibition of the biosynthesis of ergosterol, the major sterol found in fungal membranes. Binding of the triazole ring to cytochrome P450 sterol 14α-demethylase (CYP51) leads to an accumulation of 14α-methyl diols resulting in lethal disruption of fungal membrane integrity. Cytochrome P450 sterol 14α-demethylase is widely distributed among biological kingdoms and is responsible for 14α-demethylation of sterols in the biosynthesis of cholesterol in mammals, and in phytosterol and gibberellin biosynthesis in plants.
Selective inhibition of CYP51 is a key requirement for a triazole to be acceptable for timber preservation, agriculture and medicine. As an indication of relative selectivity of triazoles, the inhibitory potencies against human and yeast CYP51, expressed as a ratio of IC50 values (human/yeast) are: triadimefon 77, triadimenol 113, cyproconazole 228, tebuconazole 10, propiconazole 55 (E. R. Trosken, M. Adamska, M. Arand, J. A. Zarn, C. Patten, W. Volkel, and W. K Lutz “Comparison of lanosterol-14α-demethylase (CYP51) of human and Candida albicans for inhibition by different antifungal azoles”, 2006, Toxicology 228, 24-32), i.e. triadimefon, triadimenol and propiconazole are moderately selective for the fungal enzyme, cyproconazole is highly selective, and tebuconazole very non-selective.
It is known that in many species of fungi, plants and mammals, and in soil, triadimefon is reduced to triadimenol in what is termed an “activation” process, i.e. for many fungal species triadimenol is the fungicidally active metabolite, and that the sensitivity of individual fungal species to triadimefon is related to the extent of activation. More particularly, the sensitivity of a fungal species to triadimefon (and triadimenol) is related to extent of conversion, the stereochemistry of the triadimefon reduction reaction, i.e. which enantiomers are formed and in what relative proportions, and the sensitivity of the fungal species in question to each of the individual triadimenol enantiomers formed (see for example, M. Gasztonyi “The diastereomeric ratio in the triadimenol produced by fungal metabolism of triadimefon, and its role in fungicidal selectivity”, 1981, Pesticide Science 12, 433-438, and A. H. B. Deas, G. A. Carter, T. Clark, D. R. Clifford and C. S. James “The enantiomeric composition of triadimenol produced during metabolism of triadimefon by fungi: III. Relationship with sensitivity to triadimefon” 1986, Pesticide Biochemistry and Physiology 26, 10-21). It is well known that 1S,2R triadimenol is generally the most fungicidally active enantiomer.
Knowledge of the sensitivity of individual fungal species to triadimefon (and triadimenol) was most comprehensively disclosed by Deas et al. (cited above) who analysed the stereospecific metabolism of triadimefon and the sensitivity to each of the four triadimenol metabolites of fifteen fungal species, predominantly plant pathogens (i.e. cellulolytic) and also including two white rot decay organisms (Coriolus versicolor, now commonly known as Trametes versicolor, and Chondrostereum purpureum), and a highly cellulolytic mould active on wood (Trichoderma viride). The fungi were divided broadly into three categories to explain the sensitivity of different fungal species to triadimefon. In one category, which included Coriolus versicolor and Chondrostereum purpureum, fungi were sensitive to both triadimefon and triadimenol, and the triadimefon sensitivity appeared to be based on a high rate of triadimefon conversion and the 1S,2R enantiomer being the major contributor to the fungicidal activity among the triadimenol metabolites produced. In a further category, which included Trichoderma viride, fungi were insensitive to triadimefon and triadimenol, and converted triadimefon to forms of triadimenol dominated by enantiomers that the fungi were not sensitive to. In a still further category fungi were comparatively insensitive to triadimefon but partially sensitive to triadimenol. The latter fungi displayed either a high extent of triadimefon conversion but low sensitivity to the particular triadimenol enantiomers produced, or a low extent of conversion even though very sensitive to the enantiomers produced, both scenarios combining to produce the net effect of comparative insensitivity to triadimefon. Deas et al. also found no evidence of antagonism or synergy among the various triadimenol enantiomers.
The Pesticide Manual discloses that technical grade triadimenol comprises 70% A and 30% B. Deas et al (cited above) disclose a different enantiomeric composition (1R,2S:1S,2R:1R,2R:1S2S=21:21:29:29. Notwithstanding this difference, and considering the foregoing discussion, it should be noted that technical grade triadimenol comprises about 21-35% of the generally highly potent 1S,2R enantiomer.
In accordance with the foregoing discussion it has been acknowledged that triadimenol controls a much broader spectrum of agricultural fungal pathogens and consequently is much more widely used than triadimefon. Triadimenol also controls a number of wood degrading fungi at significantly lower concentrations than triadimefon. However, the inhibitory concentration of either active ingredient acting alone varies up to 50-fold or more when tested against a range of organisms (see, for example, EP 0254857). The minimum concentrations of active ingredients required for effective timber preservation are determined by the organisms that are least sensitive to the preservative. Thus the minimum effective dose for preservation in service is dictated by those species most resistant to a particular active ingredient.
As noted above any single active ingredient will have less effectiveness against certain fungal species. This is counteracted by combining two (or more) active ingredients to provide more effective control of fungal growth at cost effective doses against a broad spectrum of different fungal organisms.
All references, including any patents or patent applications cited in this specification are hereby incorporated by reference. The applicant makes no admission that any reference constitutes prior art—they are merely assertions by their authors and the applicant reserves the right to contest the accuracy, pertinence and domain of the cited documents. None of the documents or references constitutes an admission that they form part of the common general knowledge in Australia or in any other country.
Applicants disclose that, surprisingly, triadimenol displays an antagonistic interaction with cyproconazole with respect to fungicidal activity, whereas triadimefon displays a synergistic interaction with cyproconazole. This is entirely unexpected and surprising when it is considered that in most if not all cases where triadimefon is fungicidal against a particular fungal species, it is actually triadimenol, the metabolite of triadimefon, and is recognised as being the fungicidally active factor, not triadimefon itself. The fact that triadimenol is the active factor is recognised in the art. It is therefore totally unanticipated that triadimefon will have a synergistic interaction with cyproconazole when triadimenol, the active metabolite of triadimefon, itself displays an antagonistic interaction with cyproconazole.
The synergy observed with triadimefon and cyproconazole is all the more surprising when it is seen that triadimefon acting alone is less efficacious than triadimenol acting alone.
The net effect of this research is the enablement of improved protection of wood and glued wood products against a broad range of decay fungi and a lowering of rates of fungicidal addition to achieve such control.